1. Field of the Invention
The present invention relates to a process and a communication system, and more particularly, to a selective data transmission process and device for communication systems used in traffic engineering.
2. Discussion of Background Information
In order to assure the safe management of railroad operation, information regarding the status of the line section to be traveled must be communicated to the vehicles traveling on the rail network. Today mostly light signal systems that are controlled by a signal tower are used for this purpose, which systems indicate the closure or clearing of a line section and any other information to the engine driver. With increasing traffic density and higher speeds, the required reliability and safety of the information transmission can no longer be assured by means of this optical transmission path. Particularly under unfavorable weather conditions, the recognition of the signals is no longer assured with absolute certainty nor is the reliable association to the proper track when there is multi-track routing of rails.
For a long time, therefore, transponders have been affixed in the track, by means of which the optically signalized data are transmitted in parallel on an electrical path. These ground-based transponders can be queried over a very small air gap of a few centimeters by means of a radio station or interrogator fastened to a vehicle. It is therefore assured with a great deal of certainty that the data belonging to a track being traveled is always transmitted only to a vehicle that is traveling on this track. Since the incorrect reading of a transponder in the neighboring track is not possible for physical reasons (insufficient range of the query system), the track selectivity is thus assured.
However, the query range of the transponder system, which has been kept deliberately low to achieve the track selectivity, on the other hand has the disadvantage that the communication between the ground-based transponder and the mobile interrogator can only occur when the distance is very small. If the vehicle stops at a point at which there is a large distance between the transponder and the interrogator, e.g. in a train station, then a data inquiry is no longer possible. In order to assure the data transmission from the line section to the train over longer periods, transmission systems are therefore used, with linear antennae that extend in the direction of the track. An antenna of this kind is described, for example, in K. Bretting, Abstrahlende Hochfrequenzleitung zur Bahnsteig-Uberwachung, [Radiating High-Frequency Line for Platform Monitoring], Funkschau, Vol. 47, No. 13, 1975 Munich DE, pp. 66-68. It turns out that even with linear antenna, the overcoupling into the neighboring track cannot be prevented with absolute certainty.
In order to eliminate this uncertainty, according to a known process, a transponder is provided at both ends of the line antennae in each track, and these transponders perform the function of markers. Before a vehicle travels a line section equipped with a line antenna, it passes one of these transponders, by means of which a track identification is transmitted to the interrogator attached to the bottom of the vehicle (allocation of a track address or vehicle address). Due to the query range that has been kept deliberately low, the data of the transponder in the neighboring track cannot be received (cross-talk security). As soon as the vehicle subsequently comes into the region of the line antenna, it receives the message sent by the signal tower, which also contains a section provided with the same track identification. In a computer disposed on board the vehicle, the two messages received from the transponder and the linear antenna are compared with regard to track identification. When the track identification transmitted by the line antenna and (supposedly) by the transponder are not the same, e.g. when the travel plan is arrived at by means of undesirable physical cross-talk from the neighboring track to the vehicle, the whole message received, with the travel plan contained in it, is recognized as invalid and is discarded.
With a high degree of certainty, this known process prevents a particular travel plan, which has been received by means of cross-talk in the neighboring track, from being evaluated. With the use of the linear antenna, the phenomenon of cross-talk, however, is therefore not eliminated. The cross-talking signal can interfere with the desired signal in the track section and can make receiving impossible or can even cause bit errors. The above-described track identification, as a safety encoding, to a large extent prevents an incorrect evaluation or misinterpretation. The receipt of the useful data, however, cannot as a result be assured.